The pins or nails are driven a quarter of an inch apart, and should be properly and accurately separated, so that an even make-and-break will be the result.
It is not necessary to bore holes in the lead, but the pins or nails should be driven clear through it, so that perfect contact can be had by the metal parts coming together. Otherwise the apparatus would be useless.
Over the circle of pins a brass bridge is erected, so that the cross-strips will clear the heads of the pins. A hole is bored at the middle of the bridge so that the revolving axle will pass through it.
The axle is made from a piece of stout wire, or light rod, and near the foot of it, and about half an inch above the base-board, a disk of metal is soldered fast. A piece of spring-brass wire is attached to this disk, so that when the axle is turned the end of the wire trips from pin to pin, thus making and breaking the circuit. The upper part of the axle is bent and provided with a small wooden or porcelain knob.
One wire from the secondary coil is caught under a screw that holds one end of the brass bridge to the base; and the other to a screw, which may be placed at one corner of the block, and from which a short wire leads to the lead ring. Binding-posts may be arranged to serve the same purpose, and, of course, they are much better than the screws, because they can be easily operated by the fingers and do not require a screw-driver every time the interrupter is placed in series with an induction-coil. An interrupter on this same order may be made from a straight strip of lead with the pins driven through the middle of it. One wire from the secondary coil is made fast to the lead plate, and the end of the other wire is passed along the pins, thus making and breaking the circuit in a primitive manner.
Chapter V
ANNUNCIATORS AND BELLS
A Drum Sounder
A unique electric sounder that is sure to attract attention is in the shape of an electric-bell apparatus, with a drum sounder in place of a bell, or knockerless buzzer. [Fig. 1].
The outfit is mounted on a block four inches and a half wide and seven inches long. The cores and yoke are made as described for the [electric buzzer] ([chapter iv.]) and are wound with No. 22 cotton-insulated wire. The magnet is then strapped fast to the block by means of a hard-wood plate having a screw passed down through it; and between the coils and into the block an armature is made and mounted on a metal plate, which in turn is screwed to the block. Another block, with a contact-point, is arranged to interrupt the armature, and the series is connected as shown in the drawing [Fig. 1].